Complex Neurodevelopmental disorders (NDDs) exhibit complex etiological and genetic features, and the mutations have a fundamental role in this complexity, including common polymorphisms and rare variations in a single gene or cluster of genes. The analysis of complex NDDs have shown that the genetics has the major role in causation of such complex diseases. Interestingly both mutations and polymorphisms are involved, occurring in a single gene or clusters of genes. Likewise, a single gene variation may also be involved in multiple neurological disorders making the diagnosis of neurological diseases more difficult. Many candidate genes and chromosomal regions have been identified that are widely involved in neurological symptoms which necessitates the genotypic approach for describing the phenotype.